The transmission of a facsimile between two facsimile machines is carried out according to a data exchange protocol which makes it possible to accommodate transmission delays on the switched telephone network (STN).
Thus when one of the facsimile machines transmits data to the other it activates a time delay circuit which it deactivates if it receives a response before the time delay period runs out. In the absence of a response in the time delay period, the transmitting facsimile machine considers that the data transmitted have been lost and it carries out a new transmission according to the same process.
However, it may be the case that the two facsimile machines are connected to a network which has propagation times greater than those of the STN, such as, for example, a cellular radiotelephone network. Moreover, in this latter case, for instance, it is necessary to provide an adapter between the cable output of the facsimile machine and the radio network, which further increases the propagation time.
In such a case the propagation time back and forth between the two facsimile machines is of the order of magnitude of the time delay period and the late reception of responses frequently causes transmissions to be repeated. Moreover, it is often the case that the response returns at the moment when the data are being retransmitted so that this retransmission is interrupted and must be begun again.
There is no possibility of modifying the protocol by increasing the duration of the time delay because this would pointlessly extend the communication time in the standard case using the STN.